DESCRIPTION: The principal goal of this research is to characterize the molecular organization of the bilayer and, in particular, to provide molecule-level information concerning the nature of the heterogeneity in the lateral distribution of lipids in membranes. This heterogeneity, only recently revealed, is manifest in membranes as a loose mosaic of lipid domains if varying size. Its study is important because of the role that lipid composition plays in modulating the biological activity of membrane-associated proteins. Little is known about the organization of these domains, especially of molecular-level size "microdomains". The primary techniques to be used in this investigation are infrared and Raman spectroscopy, complemented by calorimetry. The main focus will be on the development and use of our infrared isotope method for measuring lateral microaggregation in model membranes. This technique measures microdomains of the smallest possible size, 1 to 100 molecules, a range that is inaccessible to the other techniques currently being used. At present our method can be applied only to the gel phase, but will be adapted for the fluid phase by combining it with fast-freeze methods. The lateral organization of the lipids surrounding proteins in a membrane will be investigated since such organization may play an important role in processes such as visual transduction. Other studies will focus on determining the structure and properties of microdomains in mixed lipid bilayers, on identifying the factors that lead to domain formation and, where possible, on measuring the kinetics involved. The conformational statistics of the chains in computer-modeled bilayers - which are ever more realistic and now worthy of serious scrutiny - will be critically analyzed and compared with spectroscopic measurements. Much needed vibrational spectroscopic methods for measuring disorder in the unsaturated chains in bilayers will be developed along with new spectroscopic approaches for determining bilayer structure.